Conventionally, active elements and passive elements are formed on the substrate of a semiconductor device. The active elements include transistors, diodes, etc., and the passive elements include resistors, capacitors, etc. In general, in addition to these elements, alignment markers are formed on the substrate of the semiconductor device. The alignment markers are used as patterns for position recognition for enabling automatic registration between a photomask and a wafer in manufacturing elements in wafer processes. The alignment markers are also used as patterns for automatic recognition of a chip position and coordinates on a chip in assembling processes, such as a semiconductor chip die bonding process and a wire bonding process.
In general, in wafer processes, alignment markers are recognized by an image recognition apparatus having a lens with a magnification factor 100 to 400. In contrast, in assembling processes, an image recognition apparatus has a lens with a magnification factor only 10 to 100, which is smaller than the magnification factor employed in the wafer processes. The reason is as follows.
As the lens magnification increases, the resolution becomes higher and the amount of image data to be processed increases, increasing the time taken by each alignment operation. Whereas in wafer processes, only one alignment operation needs to be performed for each process involving photoexposure, in assembling processes alignment operations need to be performed a number of times that is equal to the number of semiconductor chips. The number of times of alignment operations thus can become very large in assembling processes. Therefore, in alignment processes, the time taken by each alignment operation can to shortened by decreasing the lens magnification.
It is therefore considered that in assembling processes, an appropriate approach to increasing the alignment marker recognition accuracy is to employ alignment markers whose pattern can be recognized easily. Japanese Patent No. 2,590,711, for example, discloses undulating a metal film by forming contact holes under the metal film. This method facilitates the recognition by laser light and enables patterning of a photoresist film even if the metal film has been flattened by CMP (chemical-mechanical polishing) during the manufacturing process.
JP-A-2005-236187, for example, discloses undulating a metal film by forming contact holes in an insulating film located between an active layer and a metal layer. This method enables patterning by using the undulation of the metal film as an alignment marker even when the insulating film is formed on a substrate by a liquid phase method.
JP-A-11-135391 and JP-A-2001-326241, for example, disclose forming one or more alignment markers from a group consisting of numerals, characters, symbols, and figures by undulating one or both of a wiring layer and an insulating film in bonding pad regions. This method can increase the recognition accuracy of alignment markers and enable space saving by having certain regions shared by the bonding pads and the alignment markers. When the bonding pads are undulated, however, the contact areas between wires and the pads can be decreased, causing some difficulty in bonding the wires to the pad. Furthermore, the strength of bonding between the pads and the wires can become insufficient.
A description will now be made in a case where bonding pads and alignment markers are provided separately in the technique of the above reference JP-A-11-135391. FIG. 12 is a plan view of an alignment marker of a conventional semiconductor device. As shown in FIG. 12, the alignment marker assumes a “+” shape, for example. A metal film 105 having the “+” shape is given a “+”-shaped projection by an underlying polysilicon film 103.
FIG. 13 is a sectional view showing a sectional structure along line 13-13 of FIG. 12. As shown in FIG. 13, the alignment marker of the conventional semiconductor device is formed using a semiconductor substrate 101, such as a silicon substrate. An SiO2 layer 102 is formed on the surface of the semiconductor substrate 101. The polysilicon film 103 having a “+” shape, for example, in a plan view (see FIG. 12) is formed on part of the surface of the SiO2 layer 102. An interlayer insulating film 104 is formed on the surfaces of the SiO2 layer 102 and the polysilicon film 103. A metal film 105 made of aluminum, for example, and having a “+” shape, for example, in a plan view (see FIG. 12) is formed on part of the surface of the interlayer insulating film 104. To prevent metal corrosion due to moisture etc., a passivation film 106 is formed on the surfaces of the metal film 105 and the interlayer insulating film 104. The alignment marker of the semiconductor device shown in FIGS. 12 and 13 can increase the accuracy of its recognition and the reliability of the corrosion resistance without decreasing the strength of wire bonding.
Next, referring to FIGS. 14 and 15, a description will be made of how the alignment marker of FIGS. 12 and 13 behaves when it is illuminated with light. FIG. 14 is a sectional view showing reflection light from the alignment marker when the semiconductor device is not inclined. To recognize the alignment marker, an image recognition apparatus applies light to it. Light reflected by the metal film 105 of the alignment marker returns to the camera of the image recognition apparatus and is recognized as a pattern of the alignment marker by the image recognition apparatus.
In the conventional alignment marker, most of the surface of the metal film 105 is flat. The reflection angle of a reflection light beam from a flat region of the metal film 105 is the same as the incident angle of an incident light beam. On the other hand, for a light beam that is incident on a step region, produced by the polysilicon film 103, of the metal film 105, reflection light scattering occurs because the metal film 105 is curved there. That is, the alignment marker has regions where incident beams and reflection beams are directed regularly to prescribed directions and regions where scattering occurs due to the steps. The recognition accuracy of the alignment marker can be increased by discriminating between these different regions.
FIG. 15 is a sectional view showing reflection light from the alignment marker when the semiconductor device is inclined. As shown in FIG. 15, with the semiconductor device inclined, since most of the surface of the metal film 105 of the conventional alignment marker is flat, almost all incident beams and reflection beams are directed regularly to prescribed directions.
In each of the above-described techniques of Japanese Patent No. 2,590,711 and JP-A-2005-236187, however, steps are formed in a metal film by contact holes and these steps in the metal film are used as an alignment marker. In semiconductor devices, in general, the size of the contact hole is about 0.5 to 2.0 μm. On the other hand, as mentioned above, the magnification of the lens used in assembling processes is only about 10 to 100 so that the minimum size that can be recognized by using such a lens is one pixel, which is about 8 μm. This means that the contact holes are too small for an image recognition apparatus used in assembling processes. Hence an alignment marker that utilizes steps formed by contact holes cannot be recognized.
Furthermore, since contact holes are also formed in MOS transistors, resistors, etc., a large number of contact holes are dispersed over almost the entire surface of a semiconductor device. In an image recognition apparatus having a lens whose magnification is about 10 to 100, should contact holes be recognized, contact holes that do not belong to an alignment marker can be recognized as belonging to the alignment marker. Therefore, it is not appropriate to use such contact holes to form an alignment marker for assembling processes.
Furthermore, where a semiconductor device is inclined, almost all light that is applied to an alignment marker is reflected to a direction in which the camera of the image recognition apparatus that is applying the light does not exist. This means that the alignment marker of the semiconductor positioned at an incline can look very different from the semiconductor device that is not inclined.
Where a passivation film is formed on the metal film, light is refracted by the passivation film. Both of incident light from an image recognition apparatus and reflection light from the metal film are refracted by the passivation film, and the manner of refraction varies depending on its thickness. Therefore, if a variation occurs in the thickness of the passivation film in a wafer process, the degree of refraction of each of incident light and reflection light varies from one position to another. This means that the alignment marker configuration appears with inconsistencies.
Accordingly, there remains a need for alignment markers that can be recognized accurately even if the semiconductor device is inclined or the thickness of a passivation film is varied. Moreover, there remains a need for alignment markers that can be recognized accurately while being easily discriminated from contact holes dispersed on circuits. The present invention addresses these needs.